[banads]

>I can only think of three environments where making friends is "built-in": school, work, and church

You left out the military, but those are merely the few institutional walls that society has most oppressively forced upon us. There are countless other sports teams, game clubs, hobby communities, activist groups, charities/service organizations, hacker/maker spaces, martial arts gyms, etc. etc which we are more free to choose from, and are often more likely to enjoy. With the internet, such groups have never been easier to find.

Choosing is only an issue because we are so unused to having such agency over our own lives.